1. Field of the Invention
This invention relates to a process for producing a hightension bainitic steel sheet having excellent weldability and low-temperature toughness.
The incessant growth in demand for energy in recent years has led to a great increase in the construction of pipelines as economical means of transportation for oil and natural gas. Moreover, a good many deposits of oil and natural gas have been discovered in the coastal regions bordering on the Arctic Ocean and in Siberia. Today's pipelines, therefore, are required to transport large volumes of gas and oil over great distances. For reasons of economy, both diameter and operation pressure of pipelines have become increased. Thus, the steel sheets used for making these pipes are required to possess high strength of the API X-70 class and excellent low-temperature toughness.
Further, to ensure high efficiency in the field welding of these pipes, an automatic welding method of low heat-input has come into wide use. To prevent hardening and weld cracks in the heat-affected zone (hereinafter abbreviated to HAZ) of a welded joint, the requirements for good weldability have become ever more severe.
2. Description of the Prior Art
Most steel sheets for use in pipelines have heretofore been produced by subjecting ferrite-pearlite steel containing grain-refining and precipitation hardening elements such as of Nb and V to a method known as "controlled rolling" (hereinafter abbreviated as CR). However, the requirements for added strength, toughness and weldability of steel sheets for use in pipelines have now grown so severe that it has become exceedingly difficult to satisfy them with conventional ferrite-pearlite steel.
To satisfy these requirements, there have been developed and put to use Pearlite Reduced Steel (hereinafter abbreviated as PRS), the weldability and toughness of which is improved by decreasing the C content and consequently the pearlite content as compared with those of ordinary steel, and Acicular Ferrite Steel (hereinafter abbreviated as AF steel) having a low C content and a high Mn content and containing Nb and Mo. In the former product, however, the deficiency of strength becomes conspicuous in proportion as the wall thickness of the steel sheet increases. Moreover, the toughness and weldability of this product are not entirely satisfactory. In the latter product, although the strength and toughness are nearly satisfactory, the weldability is inferior despite the low-carbon content because the Mn and Mo contents are high and the carbon equivalent (hereinafter abbreviated as Ceq) is consequently high. Besides the two types of steel mentioned above, Bainitic Steel has been developed as a high-tension steel for use in industrial machines, although it has not yet been adapted for use in pipelines. The conventional bainitic steel possesses poor weldability and toughness in the HAZ because the bainitic transformation is accomplished by addition of large amounts of alloy elements such as Mn, Mo and B.